Interpretive Summary: Tanning of collagen, the protein in animal hides, with chromium salts has for the past century produced leather, the highest value coproduct of the meat industry. Tanning imparts heat stability, enhanced tensile properties, and resistance to microbial degradation. Although chrome-tanned leather is of the highest quality, the leather industry has long been looking for alternative tanning agents. One approach might be enzyme-based methods, which would have the advantages of a more controlled reaction and less use or generation of toxic chemicals. The research reported here is an endeavor to assist the leather industry in the rational design of enzymatic tanning methods. Our previously reported computer model of a collagen molecular complex has now been modified to focus on the potential for enzymatic crosslinking. This model may now be used in the design of tanning processes that use biochemical as well as chemical agents. The introduction of biochemical tannages would benefit domestic tanneries by allowing them to develop new classes of products and reduce waste disposal costs.

Technical Abstract:
Type I collagen, the primary substrate for the tanning industry, is comprised of two alpha-1(I) and one alpha-2(I) chains containing more than 3000 amino acid residues. More than 95% of each protein chain of the fibrillar collagens has repeating tripeptide sequence, Glycine-X-Y where X and Y are often proline and hydroxyproline. This amino acid sequence imposes a rigid helical structure that coils into a triple helix and becomes the basis for supramolecular structures that give shape to organisms. Short extensions (telopeptides), without the tripeptide repeat, at the amino and carboxyl termini of the triple helix form the remaining 4% to 5% of the protein. This telopeptide region is highly flexible, allowing reactive side-chains such as amines and carboxyls to assume conformations favorable for interactions with adjacent triple helix chains. Published reports of possible conformations for the carboxy terminal telopeptides of type I collagen were based on spectroscopic studies of the isolated peptides or on analysis of their amino acid sequences. The ERRC collagen microfibril model makes it possible to evaluate these predictions in the context of a supramolecular collagen structure. A conformation based on the polyproline helix and a similar conformation predicted from NMR studies could be located within the gap region of the microfibril and would be suitable for interactions with neighboring triple helices. Conformations developed from solution studies of isolated peptides resulted in a telopeptide chain protruding out of the microfibril and toward more distant helices.